水素　　

WordNet

a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe (同)H, atomic number 1

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『水素』(化学記号は『H』)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/11/18 23:27:45」(JST)

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「Hydrogen」はこの項目へ転送されています。ソフトウェアについては「Hydrogen (ソフトウェア)」をご覧ください。

この項目では、元素について記述しています。ゲームソフトウェアについては「水素 (ソフトウェア)」をご覧ください。

水素原子における電子の軌道はエネルギーの固有関数である。

水素（すいそ、羅: Hydrogenium、英: hydrogen）は、原子番号 1 、原子量1.00794^[1]の元素である。元素記号は H。非金属元素の一つ。元素およびガス状分子の中で最も軽く^[2]、また宇宙で最も数が多く^[1]、珪素量を10⁶とした際の比率は2.79×10¹⁰である^[4]。地球上では水や有機化合物の構成要素として存在する。

一般に「水素」という場合は、水素の単体である水素分子（水素ガス） H₂ を示すことが多い。水素分子は常温・常圧では無色無臭の気体で、とても軽く、非常に燃焼・爆発しやすいといった特徴を持つ。日本では、高圧ガス保安法容器保安規則により、赤いボンベに保管するように決められている^[2]。

1 分布
2 歴史
3 同位体
4 水素分子
- 4.1 オルト水素とパラ水素
5 金属水素
- 5.1 超伝導の可能性
6 水素分子の生産
7 用途
- 7.1 代表的な用途
- 7.2 エネルギー利用
  - 7.2.1 燃料電池
  - 7.2.2 貯蔵技術
8 化学的性質
- 8.1 水素化物
- 8.2 水素イオンと水素化物イオン
- 8.3 ヒドロン・プロトンとヒドロニウムイオン
- 8.4 ヒドリド
- 8.5 周期表上の位置
9 惑星の水素散逸
10 水素と似た粒子
11 脚注
12 参考文献
13 関連項目
- 13.1 物理学
- 13.2 化学
- 13.3 利用
14 外部リンク

分布[編集]

水素は宇宙で最も豊富にある元素であり、（ダークマターとダークエネルギーを除いた）宇宙の質量の3/4を占め^[5]、総量数比では全原子の 90 % 以上となる^[6]。これらのほとんどは星間ガスや銀河間ガス、恒星あるいは木星型惑星の構成物として存在している。地球表面の元素数では酸素・珪素に次いで三番目に多い^[1]が、水素は質量が小さいため、質量パーセントで表すクラーク数では9番目となる。ほとんどは海水^[1]の状態で存在し、単体の水素分子状態では天然ガスの中にわずかに含まれる程度である。地球の大気中での濃度は 1 ppm 以下とほとんど存在していない。

水素原子は宇宙が誕生してから約38万年後^[7]に初めてできたとされている。それまでは陽子と電子がバラバラのプラズマ状態で光は宇宙空間を直進できなかったが、電子と陽子が結合することにより宇宙空間を散乱されずに進めるようになった。これを宇宙の晴れ上がりと言う。

宇宙における主系列星のエネルギー放射のほとんどはプラズマとなった4個の水素原子核がヘリウムへ核融合する反応によるもので、比較的軽い星では陽子-陽子連鎖反応、重い星ではCNOサイクルという過程を経てエネルギーを発生させている。水素原子はいずれの核融合反応においてもこれを起こす担い手である^[8]。

歴史[編集]

水素は水の主成分であるため、日本語の「水素」のみならず、欧米語圏でも「水を生む物」という語で呼ばれて来た。英語の「hydrogen」や、仏語の「hydrogène」（日本語読み：イドロジェーヌ）は、ギリシア語の ὕδωρ（水。ラテン文字表記：hydôr）と γννεν（発生。ラテン文字表記：gennen）を合わせた語で、「水を生む物」を意味する合成語である^[1]。同様に、独語でも「Wasserstoff」という。

水素を気体として分離して発見したのは1766年のヘンリー・キャヴェンディッシュであり、アントワーヌ・ラヴォアジエが1783年に hydrogèneと命名した^[1]。ただし、1671年にはロバート・ボイルが鉄と希硝酸を反応させて生じる気体が可燃性であることを記録している^[1]。

一方、中国語では、化合物の「水」と元素の「水素」が別の漢字で区別されており、水素には「氫」（中国語読み：チン。日本語読み：けい）という字を充てる。

同位体[編集]

水素の同位体左からそれぞれ水素、重水素、三重水素。図中の赤い丸は陽子を、黒い丸は中性子を、そして青い丸は電子を表している。

詳細は「水素の同位体」を参照

水素には、水素（軽水素）¹H 、重水素 ²H （デュウテリウム、ジューテリウム^[9]、略号D）、三重水素 ³H （トリチウム、略号T）の三つの同位体が知られている^[1]。このうち、最も軽い ¹H は、一つの陽子と一つの電子のみによって構成されており、原子の中で中性子を持たない核種の1つである。存在が確認されている中で他に中性子を持たない核種はリチウム3のみである。それぞれの同位体は質量の差が2倍・3倍となり、性質の違いも大きい。例えばD₂はH₂よりも融点や沸点が高くなり、溶融潜熱は倍近くに、蒸気圧は1/10近くとなる^[10]。2013年現在、より重い同位体は水素4から水素7までが確認されている。最も重い水素7（原子核は陽子1、中性子6よりなる）はヘリウム10を軽水素に衝突させることで合成されている。質量数が4以上のものは寿命が極めて短く、たとえば水素7では半減期が23ヨクト秒ほどしかない^[11]。

水素の同位体は、それぞれの特徴を有効に活かした使い方をされる。重水素は原子核反応での用途で、中性子の減速に使用され、化学や生物学では同位体効果の研究、医療では診断薬の追跡^[9]に使用されている。また、三重水素は原子炉内で生成され、水素爆弾の反応物質や核融合燃料、放射性を利用したバイオテクノロジー分野でのトレーサーや発光塗料の励起源として使用されている。

水素分子[編集]

水素の線スペクトル例。バルマー系列と呼ばれる。

水素分子は、常温常圧では無色無臭の気体として存在する、分子式 H₂ で表される単体である。分子量2.016、融点 −259.2 ℃（常圧）、沸点 −252.6 ℃（常圧）、密度 0.0899 g/L、比重 0.0695（空気を1として）、臨界圧力12.80気圧、水への溶解度0.021 mL/mL水（0 ℃）。最も軽い気体である。原子間距離は 0.074 nm、結合エネルギーはおよそ 104 kcal/mol^[2]。

水素分子は常温で安定であり、フッ素以外とは反応を起こさない。しかし何かしらの外部要因があればその限りではなく、例えば光がある状態では塩素と激しい反応を起こす^[10]^[2]。また水素と酸素を混合したものに火を付けると起こす激しい爆発（水素爆鳴気）は、混合比下限は4.65 %、上限は93.3 %であり、空気との混合では4.1 %〜74.2 %となり、これはアセチレンに次ぐ広い爆発限界の範囲を持つ^[2]。

ガス密度が低い水素は早い速度で拡散する性質を持ち、また燃焼時の伝播も早い。そのため、ガス漏れを起こしやすい傾向にある^[2]。原子径の小ささから、金属材料に侵入し機械的特性を低下させる（水素脆化）傾向が強い。これは高温高圧環境下で顕著となり、封入容器の材質には注意を払う必要がある。−250 ℃以下で液化させると体積は1/800となり、しかも軽いため低温貯蔵性には優れる^[12]。

ガス惑星の内部など非常に高い圧力下では性質が変わり、液状の金属になると考えられている。逆に宇宙空間など非常に圧力が低い場合、H₂⁺やH₃⁺、単独の水素原子などの状態も観測されている。H₂ 分子形状の雲は星の形成などに関係あると考えられている。

オルト水素とパラ水素[編集]

水素分子は、それぞれの原子核（プロトン）の核スピンの配向により、オルト（ortho）とパラ（para）の2種類の異性体が存在する^[10]。オルト水素は、互いの原子核のスピンの向きが平行で、パラ水素ではスピンの向きが反平行である。この2つは、化学的性質に違いがないが、物理的性質（比熱や熱伝導率など）がかなり異なる。これは内部エネルギーにある差によるもので、パラ水素側が低い^[10]。統計的な重みが大きいほうをオルトと呼ぶ。

常温以上では、オルト水素とパラ水素の存在比はおよそ 3:1 である。低温になるほどパラ水素の存在比が増し、絶対零度付近ではほぼ 100 % パラ水素となる^[10]。オルト‐パラ変換を起こす触媒は、活性炭や鉄などの金属の一部、常磁性物質またはイオンなどがある^[10]。

金属水素[編集]

詳細は「金属水素」を参照

高い圧力下において金属化すると考えられている水素は、実際に1996年にローレンス・リバモア国立研究所のグループが、140 GPa（1 GPa = 約1万気圧）, 数千℃という状態で、100万分の1秒以下という短寿命ではあるが、液体の金属水素を観測したと報告している^[13]^[14]。しかしながら、2006年現在、数百GPaのオーダーで圧力を加える実験が行われているものの、固体の金属水素の観測はされていない。

励起状態の水素が金属化すると極めて強力な爆薬になるとの理論計算が行われ、電子励起爆薬として研究されている。この理論では圧力だけでは不十分であり、水素を励起状態にして圧力をかければ金属化するとしている。

超伝導の可能性[編集]

金属化そのものが達成されていないためにその真偽は未だ不明であるが、金属化した水素は室温超伝導を達成するのではないかという予想がある^[15]。この可能性の傍証として、周期表で水素のすぐ下のリチウムは、30 GPa 以上という超高圧下で超伝導状態となることが示されている。リチウムの超伝導への転移温度は圧力 48 GPaで20 K程度であるが、この数字は単体元素のものとしては高い部類に入り、いくつかの例外を除けば一般に軽い元素ほど転移温度は高くなるため、最も軽い元素である水素は、より高い転移温度を持つ可能性が十分ある。

木星型惑星（木星・土星）の深部は非常に高い圧力になっており、液体金属水素が観測された条件と似ている。木星型惑星を構成する最も主要な元素の一つである水素は、この状況下では金属化している可能性があり、惑星の磁場との関わりも指摘されている^[16]。

水素分子の生産[編集]

赤い水素ボンベ

工業的には、炭化水素の水蒸気改質や部分酸化の副生成物として大量に生産される（炭化水素ガス分解法）。硫黄酸化物を除いたパラフィン類やエチレン・プロピレンなどを440℃の環境下でニッケルを触媒としながら水蒸気と反応させ、粗ガスを得る^[2]。

C_nH₂n₊₂ ＋ nH₂O → nCO ＋ (2n+1)H₂
C_nH₂n₊₂ ＋ 2nH₂O → nCO ＋ (4n+1)H₂

副生される一酸化炭素は水蒸気と反応し二酸化炭素と水素ガスとなる。後にガーボトール法にて二酸化炭素を除去し、水素ガスが得られる^[2]。粗ガスの精製には、圧縮した上で苛性ソーダ洗浄を行い、熱交換器にて重いガス類を液化除去する方法（液化窒素洗浄法）もある^[2]。

また、ソーダ工業や製塩業において海水電気分解の副生品として発生する水素が利用されることもある。現在のところ、水素ガスはメタンを主成分とする天然ガスと水から、触媒を用いた水蒸気改質によって生産する方法が主流である。日本国内における2008年度の水素の生産量は 534,810×10³ m³、工業消費量は 309,645×10³ m³である^[17]。

水素分子（水素ガス）を生じる化学反応は多岐に渡る。古典的には実験室において小規模に生成する場合、亜鉛やアルミニウムなど水素よりもイオン化傾向の大きい金属に希硫酸を加えて発生させる方法が知られている（キップの装置）。あるいは水酸化ナトリウムや硫酸などを添加して電導性を増した水や、食塩水を電気分解して陰極から発生させることもできる。実験室レベルにおいては工業的に生産されたガスボンベ入りの水素ガスを利用する。

用途[編集]

スペースシャトルのメインエンジン。1機を打ち上げるには150万リットルの液体水素が使われる^[4]。

代表的な用途[編集]

原料 - アンモニアの製造（ハーバー・ボッシュ法）^[10]の他、塩素ガスと混合し光を当てて反応させる塩酸の製造^[1]、油脂に添加して炭素同士の二重結合数を減らし固体化する改質（トウモロコシ油や綿実油のマーガリン化など）^[1]、脱硫など、多方面に利用されている。
還元剤 - 金属鉱石（酸化物）の還元^[1]、ニトロベンゼンを還元しアニリンの製造、ナイロン66製造におけるベンゼンの触媒還元、一酸化炭素を還元するメチルアルコール合成などに使われる^[10]。
燃料 - 燃やしても水以外の排出物、例えば、粒子状物質や二酸化炭素などの排気ガスを出さないことから、代替エネルギーとして期待されている^[12]。ただし、燃焼条件により窒素酸化物が生成する場合はある。内燃機関の燃料として水素燃料エンジンを積んだ水素自動車が発売されている他、ロケットの燃料や燃料電池に使用されている。

上記で述べたように、水素ガスの生産は原料を化石燃料に依存しており、水蒸気改質により発生する一酸化炭素などのうち化成品に利用されない過剰分や燃料として利用される炭化水素は二酸化炭素として環境中に放出される。水素の原料が化石燃料である限りにおいては、水素を化石燃料の代替として利用してもそのまま化石燃料の消費量が削減されたり二酸化炭素の発生が抑えられたりすることにはならない。

浮揚ガス - 1リットルの水素を詰めた風船は1.2グラムの質量を浮揚させる^[1]。この性質から気球や飛行船などに用いられる。
冷却剤 - 液体水素は超伝導現象を含む低温学の調査に使用される。また、発電所では、水素ガスを冷却媒体として用いている発電機もある。これは空気よりも熱伝導率が7倍と高く^[1]風損が少ないためである。水素ガスが漏れないようにするため、水素ガス圧力よりも高い圧力の油を流し遮蔽する。
洗浄 - 工業分野では、半導体の洗浄はRCA洗浄が主流でアンモニアや塩酸フッ化物が用いられるが、その代替として水素を水に溶かし込んだ水溶液は排水処理の面で環境負荷が低く^[18]、半導体の基板表面の微粒子除去・洗浄に用いられる^[19]。
溶接 - 水素分子を一旦二つの水素原子に解離させ、それを再結合させると多量の熱を発生する。これを利用した金属溶接法がある^[10]。
その他 - テクニカルダイビングや軍隊などで大深度潜水時の使用が試みられたが、同時に酸素も用いられるために爆発の可能性が付きまとうなど、危険であるため使用されていない。

エネルギー利用[編集]

水素は、エネルギー変換効率の高い点、先述のとおり化石燃料を使って製造した水素もあるものの、水の電気分解やバイオマス・ごみ利用などを利用すれば化石燃料に拠らないで製造することも可能である点、燃焼後に二酸化炭素を排出しない点などから、将来性の高いエネルギーの輸送及び貯蔵手段として期待される^[12]。

水素は様々な利用法が考えられている。まず水素を言わば「電池」として利用することも考えられている。鉛蓄電池、リチウム電池、NAS電池など、比較的大きな容量の充電が可能な電池が色々と開発されてきたものの、それでも電気エネルギーは貯めておくのが比較的困難なエネルギーとして知られている。そこで、必要以上の電力が得られる時に水を電気分解して生産した水素を貯蔵し、電力が必要となった時に貯蔵しておいた水素を使って発電を行うのである。必要以上の電力が得られる時に水をポンプで汲み上げて水の位置エネルギーとして電気エネルギーを貯める揚水発電はすでに実用化されているが、それと同様に電力需要のピーク時に対応する手法の一つとして水素は利用できる。

他にも太陽光発電や風力発電といった発電法のように、発電量が比較的自然条件に左右されやすいものの、十分な発電量が得られる時に水の電気分解を行って水素を貯蔵するという方法で、これらの発電量の不安定さを解消する方法が考えられている。

他にも水素を電力の輸送手段として利用することも考えられている。長距離の送電を行うと送電線の抵抗などの関係で送電によるエネルギーの損失（送電ロス）が多くなる。小水力発電や火力発電や比較的低温の熱源を利用した発電法などのように、電力需要の多い都市の近くに発電所を立地できる場合は送電ロスの問題もあまりない。しかし必要に応じて変圧を行うなど送電ロスを少なくする工夫は行われているものの、2011年現在、送電ロス無しに長距離を送電する手法は実用化されていない。このためいわゆる自然エネルギーを利用した発電法に限らず、あらゆるエネルギーを利用した発電法において電力の供給地と需要地とが離れている場合には、どうしても送電ロスの問題が避けられない。ここで水素として輸送すれば、水素を逃がさなければ輸送中の水素のロスは発生しない。ただし水素を輸送する手段によって消費されるエネルギー（例えば自動車で輸送すれば燃料が消費される）もあるので、どうしてもエネルギーのロスは発生してしまうという問題は残る。しかし燃料電池を用いることで、燃料電池で電力を作ると同時に発生する熱も利用可能となるという別な利点も生ずる。

他に水素は液化すると体積が小さくなるため小さなタンクで持ち運びが可能という利点もある。このため水素と燃料電池を組み合わせることで、電力が必要な場所に送電線を利用して電力供給しにくい場所に電力を供給するという利用法も検討されている。例えば自動車や船舶などに向けての電力供給である。

また最近ではマグネシウムと水を反応させて水素を作り出す方法も開発されている。マグネシウムと水が反応して発生する水素の他、反応時の熱もエネルギー源として利用できる。最大の課題は使用後のマグネシウムの還元処理で、太陽光などから変換したレーザー照射による高温により還元する方法が考えられている。他に燃料電池の燃料としての水素の利用はよく知られているが、コンバインドサイクル発電などに利用することも考えられている。

燃料電池[編集]

詳細は「燃料電池」を参照

空気中の酸素と反応させて水を生成しながら発電する水素‐酸素型燃料電池は19世紀中ごろには実験的に成功しており、20世紀の宇宙開発を通じて技術検討が進んだ。燃料電池は発電効率が35–60 %と高く、発熱エネルギーを回収すれば80 %まで高めることができる。環境負荷も低い。燃料にはメタノールを用いるタイプもあるが、水素ガスを利用するものでは自動車への積載を念頭に置いた固体高分子形燃料電池（PEFC）が有力視されており、電解質分離膜や電極劣化の抑制など技術開発が進められている^[12]。また宇宙船では燃料電池から得られる電力の他に、同時に生成される水の利用も行われることがある。

貯蔵技術[編集]

水素をエネルギー利用する上での課題のひとつには、ガス状水素を貯蔵する際の問題がある。既述のように空気との混合4.1 %～74.2 %という広い爆発限界の範囲を持つために、漏出しないようにする技術が必要となるわけだが、水素は原子半径が小さいために容器を透過したり劣化させたりするので、他の元素や他の燃料を貯蔵するのとは勝手が違ってくる。2002年2月に発足した「燃料電池プロジェクト・チーム」の報告では、自動車に積載しガソリン相当の500 km以上走行が可能な水素貯蔵を目標に据えた。これに相当する水素ガスは「5 kg」であり、常温常圧下では56,000 Lに相当する^[12]。

従来からの手法では、高圧化と液体化がある。水素は金属脆化を起こすため、特に高圧ガスを密閉するにはアルミニウム‐マグネシウム‐シリコン合金をファイバー強化したものが開発されているが、日本の高圧ガス保安法が定める上限の350気圧では実用的に自動車積載が可能なガス量は3.5 kgに止まり、5 kgを実現するためには安全に700気圧相当を密封できる容器が検討されている。液体化も同様な問題を解決する必要があり、オーステナイト系ステンレス鋼やアルミニウム合金・チタン合金等を素材に検討が進む。しかし、高圧化や液体化には密封する際にも加圧や冷却などでエネルギーを消費してしまう点も課題として残る^[12]。

水素を貯蔵する物質には金属類である水素吸蔵合金と、無機・有機物質が提案されており、いずれも水素化物を作り効率的に水素を捕まえることが出来る。水素吸蔵合金は、ファンデルワールス力（分子間力の1種）で表面に吸着（物理吸着）させた水素分子を原子に解離（解離吸着、化学吸着）し、水素化合物を反応生成しながら合金の格子内に水素原子を拡散させる。取り出すには加熱または合金周囲の水素ガス量を減らすことで水素化物が分解しガスが放出される。必要な温度は通常50 ℃であり高くとも250℃位、圧力も常圧から100気圧程度までであり、水素ガスの体積を1000分の1に収めることが出来る。課題は合金と水素の重量比にあり、現状では「5 kg」の水素を吸蔵するための合金重量は170–500 kg程度が必要になる^[12]。この他、イオン結合を主とする錯体水素化物や、アンモニアボランなども水素吸蔵性能を持つ物質として研究されている^[12]。

化学的性質[編集]

水素化物[編集]

詳細は「水素化合物」を参照

元素の水素化物
化学式	IUPAC組織名^[20]	慣用名
BH₃	ボラン	ホウ化水素
CH₄	カルバン	メタン
NH₃	アザン	アンモニア
H₂O	オキシダン	水
HF	フッ化水素
AlH₃	アラン	水素化アルミニウム
SiH₄	シラン	水素化ケイ素
PH₃	ホスファン	ホスフィンリン化水素
H₂S	スルファン	硫化水素
HCl	塩化水素
GaH₃	ガラン
GeH₄	ゲルマン	水素化ゲルマニウム
AsH₃	アルサン	アルシン
H₂Se	セラン	セレン化水素
HBr	臭化水素
SnH₄	スタナン	水素化スズ
SbH₃	スチバン	スチビン
H₂Te	テラン	テルル化水素
HI	ヨウ化水素
PbH₄	プルンバン	水素化鉛
BiH₃	ビスムタン	ビスムチン

水素は電気陰性度が2.2であり、酸化剤としても還元剤としても働く。このため非金属元素とも金属元素とも親和しやすい。例えば、水素と酸素が化合するときには還元剤として働き爆発的な燃焼と共に水 H₂O を生じる。ナトリウムと水素との反応では酸化剤として働き、水素化ナトリウム NaH を生じる。このような水素と他の元素が化合した物質を水素化物という^[21]。

水素化物の結合には、イオン結合型・共有結合型の他に、パラジウム水素化物などの侵入型固溶体（侵入型化合物）と呼ばれる三種類の形態がある^[21]。イオン結合型の化合物の中では、水素は H⁻ イオン（ヒドリドイオン）として存在する。共有結合型は電気陰性度が高いPブロック元素と電子を共有して化合する^[21]。侵入型固溶体は一種の合金であり、水素原子は金属原子の隙間にはまり込むように存在している。このため、容易かつ可逆的に水素を吸収・放出することが出来、水素吸蔵合金に利用される。なお、高性能な水素吸蔵合金中の水素原子の密度は、液体水素のそれに匹敵する。

一方、より電気陰性度の大きい元素との化合物では水素は H⁺ イオンとなる。水中で水素イオンを生じる物質が狭義の酸である。水溶液中では水素イオンは、H⁺（ヒドロン）ではなく、水分子とくっついて H₃O⁺（オキソニウムイオン）として振舞う。

水素はまた、炭素と結合することで、様々な有機化合物を形成する。ほとんど全ての有機化合物は構成原子に水素を含む（下に例を示す）。

メタン (CH₄)
エタノール (C₂H₅OH)
ベンゼン (C₆H₆)

おもな元素の水素化物の化学式と国際純正応用化学連合 (IUPAC) による組織名、および（存在するものは）慣用名を右表に示す。

水素イオンと水素化物イオン[編集]

水素のイオンには、陽イオンである水素イオン（hydron, ヒドロン又はハイドロン）と、陰イオンの水素化物イオン（hydride,ヒドリド又はハイドライド）とが存在する。¹H⁺ はプロトン（陽子）そのものであるが、一般に水素は同位体混合物なので、水素の陽イオンに対する呼称としてはヒドロンが正確である（すなわちヒドロンは H⁺、D⁺、T⁺ の総称である）。しかし、化学の領域において単に「プロトン」と呼ぶ際は水素イオンを指し示していると考えて差し支えはない。

水素イオンの濃度 [H⁺]は酸性度を定量的に表す指標として用いられ、mol/L（モル毎リットル）単位で表した水素イオンの濃度の数値の対数に負号をつけた値を水素イオン指数 (pH) で表す。水中の [H⁺]濃度は1から10⁻¹⁴ mol/L程度の広い範囲を取り、pHでは0から14程度となる。中性の水には約10⁻⁷ mol/L の水素イオンが存在し、pHは約7となる^[1]。

ヒドロン・プロトンとヒドロニウムイオン[編集]

H⁺ であれ D⁺ であれ、ヒドロンは電子殻を持たないむき出しの原子核であるため、化学的にはファンデルワールス半径を持たない正の点電荷の様に振る舞う。それゆえ通常は単独で存在せず、溶媒など他の分子の電子殻と結合したヒドロニウムイオン (hydronium ion) として存在する。水素のイオン化エネルギーは1131 kJ mol⁻¹、遊離状態の水素イオンの水和エネルギーは1091 kJ mol⁻¹と見積もられており^[21]、これは高い電子密度に起因する、水分子との高い親和力を示すものである。

H⁺ (g) → H⁺ (aq)

極性溶媒中では、水、アルコール、エーテルなどの酸素原子の電子殻と結合している場合が多いので、ヒドロニウムイオンと言う代わりにオキソニウムイオン (oxonium ion) と呼ばれることも多い。あるいは超強酸など極限状態においては単独で挙動するプロトンも観測されている。

また、アレニウスの定義ではヒドロンは酸の本体である。酸としてのプロトンの性質は記事オキソニウムあるいは記事酸と塩基に詳しい。

ヒドリド[編集]

アルカリ金属、アルカリ土類金属あるいは第13族・14族元素（共有結合性が強い）などの、電気的に陽性な元素の水素化物が電離するとき、ヒドリド (hydride, H⁻) が生成する。水素化物イオンとも呼ばれる。ヒドリドは K 殻が閉殻した電子配置を持ちヘリウムと等電子的であるために、一定の大きさを持ったイオンとして振舞う点でヒドロン（水素カチオン）とは異なる。実際、ヒドリドはフッ素アニオンよりもイオン半径が大きいように振舞う。

ヒドリドは極めて弱い酸でもある水素分子 (pK_a = 35) の共役塩基であるので、強塩基として振舞う。

ヒドリドは塩基として作用する場合と還元剤として作用する場合があるが、それは金属と還元をうける化合物との組み合わせにより変化する。ヒドリドの標準酸化還元電位は−2.25 Vと見積もられている。

H₂ (g) + 2 e⁻ → 2 H⁻ (aq)

周期表上の位置[編集]

一般的な周期表では水素はアルカリ金属の上に配置されるが、2006年に周期表における水素の位置を変更すべきなのではないか^[22]とする論文がIUPACに提出され、公式雑誌に掲載された^[23]。

惑星の水素散逸[編集]

宇宙空間に散逸する地球の大気は少ないが、それでも1秒あたり水素が3 kg、ヘリウムが50 gずつ放出されている。これは大気が薄く原子や分子の速度が減速されずに宇宙へ飛び出すジーンズエスケープやイオン状態の荷電粒子が地球磁場に沿って脱出するプロセスがある。なお、加熱された粒子がまとまって流出するハイドロダイナミックエスケープや太陽風が持ち去るスパッタリングは現在の地球では起きていないが、地球誕生直後はこの作用によって水素が大量に散逸したと考えられる^[24]。

固有磁場を持たない金星は現在でもハイドロダイナミックエスケープやスパッタリングが続き、地表には比較的重いため残った酸素や炭素が作る二酸化炭素が大気のほとんどを占め、水が無い非常に乾燥した状態にある。火星も軽い水素を中心に散逸し、かろうじて氷となった水が極部分の土中に残るに止まる^[24]。

水素と似た粒子[編集]

水素原子は非常に簡単な構造をしているため、水素の陽子または電子を別の粒子に置き換えた粒子は多数存在する。これらは水素と似たような化学反応を起こすものもある。

反水素：陽子を反陽子に、電子を陽電子に置き換えた粒子。
ポジトロニウム：陽子を陽電子に置き換えた粒子。
プロトニウム：電子を反陽子に置き換えた粒子。
ミューオニウム：陽子を反ミュー粒子に置き換えた粒子。
K中間子水素：電子を負電荷のK中間子に置き換えた粒子。

脚注[編集]

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t ^u ^v ^w ^x ^y 桜井弘「水素」『元素111の新知識』講談社、1997年、30-34頁。ISBN 4-06-257192-7。
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j 「水素」『12996の化学商品』化学工業日報、1996年、233-234頁。ISBN 4-87326-204-6。
^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
^ ^a ^b 『ニュートン別冊周期表第2冊付録周期表』ニュートンプレス、2010年。ISBN 978-4-315-51876-4。
^ Palmer, D. (1997年9月13日). “Hydrogen in the Universe”. NASA. 2010年5月8日閲覧。
^ Anders, Edward; Nicolas Grevesse (1989). “Abundances of the Elements-Meteoritic and Solar”. Geochimica et Cosmochimica Acta 53: 197.
^ クリエイティブ・スイート『宇宙の秘密』 PHP研究所、2009年、22頁。ISBN 978-4-569-67352-3。
^ 西尾正則. “宇宙科学入門第7回資料 (PDF)”. 鹿児島大学理学部. 2010年5月9日閲覧。
^ ^a ^b 「【重水素】」『12996の化学商品』化学工業日報、1996年、234-235頁。ISBN 4-87326-204-6。
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ J.D.Lee 「3.元素の一般的性質水素」『リー無機化学』浜口博、菅野等訳、東京化学同人、1982年、119-123頁。ISBN 4-8079-0185-0。
^ G. Audi et al. (2003). “The Nubase evaluation of nuclear and decay properties”. Nuclear Physics A (Elsevier) 729 (1): 27. doi:10.1016/j.nuclphysa.2003.11.001.
^ ^a ^b ^c ^d ^e ^f ^g ^h 東北大学金属材料研究所「8.燃料電池と水素貯蔵材料」『金属材料の最前線』講談社、2009年、241-259頁。ISBN 978-4-06-257643-7。
^ Weir, S. T.; Mitchell, A. C.; Nellis, W. J. (1996). "Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 Mbar)". Phys. Rev. Lett. 76: 1860–1863. doi:10.1103/PhysRevLett.76.1860
^ W・J・ネリス「金属水素を作る」（日経サイエンスのページ）[1]
^ “超高圧分野の研究内容”. 大阪大学極限量子科学研究センター. 2010年5月9日閲覧。
^ “木星”. 福岡教育大学金光研究室. 2010年5月9日閲覧。
^ 化学工業統計月報 - 経済産業省
^ 「水の活性化と機能水-表面処理における各種対策について」『鍍金の世界』41(4)[2008.4]、52～56頁。
^ 黒部洋（栗田工業株式会社）「機能水の製造方法および洗浄効果オプト・半導体デバイスにおけるウェットプロセスの技術トレンド（薬品・機能水編）」『マテリアルステージ』7(10)[2008.1]、40～43頁。
^ IUPAC Nomenclature of Organic Chemistry /Recommendations 1979 and Recommendations 1993 by ACD Lab. Inc.）
^ ^a ^b ^c ^d J.D.Lee 「3.元素の一般的性質水素化物」『リー無機化学』浜口博、菅野等訳、東京化学同人、1982年、123-126頁。ISBN 4-8079-0185-0。
^ ハロゲンに近い性質を持つため、1周期系列と17族の位置に変更すべきというもの。
^ 玉尾皓平、桜井弘、福山秀敏『完全図解周期表 - 自然界のしくみを理解する第1歩』ニュートンプレス〈ニュートンムック - サイエンステキストシリーズ〉、2006年。ISBN 978-4315517897。
^ ^a ^b 「惑星の顔を決める大気流出」『見えてきた太陽系の起源と進化』日経サイエンス〈別冊日経サイエンス〉、2009年、134-142頁。ISBN 978-4-532-51167-8。

参考文献[編集]

Anders, Edward, Nicolas Grevesse (1989). "Abundances of the Elements-Meteoritic and Solar". Geochimica et Cosmochimica Acta 53: 197
Lee, J.D. 『リー無機化学』浜口博、菅野等訳、東京化学同人、1982年 ISBN 4-8079-0185-0
桜井弘、『元素111の新知識』講談社、1997年 ISBN 4-06-257192-7
玉尾皓平、桜井弘、福山秀敏『完全図解周期表 - 自然界のしくみを理解する第1歩』ニュートンプレス〈ニュートンムック - サイエンステキストシリーズ〉、2006年 ISBN 978-4315517897
東北大学金属材料研究所『金属材料の最前線』講談社、2009年 ISBN 978-4-06-257643-7
Weir, S. T.; Mitchell, A. C.; Nellis, W. J. (1996). "Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 Mbar)". Phys. Rev. Lett. 76
『12996の化学商品』化学工業日報、1996年 ISBN 4-87326-204-6
『ニュートン別冊周期表第2冊付録周期表』ニュートンプレス、2010年 ISBN 978-4-315-51876-4
『見えてきた太陽系の起源と進化』日経サイエンス〈別冊日経サイエンス〉、2009年 ISBN 978-4-532-51167-8

外部リンク[編集]

「Hydrogen」 - Encyclopedia of Earthにある「水素」についての項目（英語）。
国際化学物質安全性カード水素 - 国立医薬品食品衛生研究所

wiki en

[Wiki en表示]

This article is about the chemistry of hydrogen. For the physics of atomic hydrogen, see Hydrogen atom. For other meanings, see Hydrogen (disambiguation).

Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u, hydrogen is the lightest element and its monatomic form (H) is the most abundant chemical substance, constituting roughly 75% of the Universe's baryonic mass.^[7]^{[note 1]} Non-remnant stars are mainly composed of hydrogen in its plasma state.

At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H₂. Most of the hydrogen on Earth is in molecules such as water and organic compounds because hydrogen readily forms covalent compounds with most non-metallic elements.

Hydrogen plays a particularly important role in acid–base chemistry with many reactions exchanging protons between soluble molecules. In ionic compounds, it can take a negative charge (an anion known as a hydride and written as H⁻), or as a positively charged species H⁺. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species.

The most common isotope of hydrogen is protium (name rarely used, symbol ¹H) with a single proton and no neutrons. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.

Hydrogen gas was first artificially produced in the early 16th century, via the mixing of metals with acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance,^[8] and that it produces water when burned, a property which later gave it its name: in Greek, hydrogen means "water-former".

Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water.^[9] Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.

Hydrogen is a concern in metallurgy as it can embrittle many metals,^[10] complicating the design of pipelines and storage tanks.^[11]

1 Properties
- 1.1 Combustion
- 1.2 Electron energy levels
- 1.3 Elemental molecular forms
- 1.4 Phases
- 1.5 Compounds
  - 1.5.1 Covalent and organic compounds
  - 1.5.2 Hydrides
  - 1.5.3 Protons and acids
- 1.6 Isotopes
2 History
- 2.1 Discovery and use
- 2.2 Role in quantum theory
3 Natural occurrence
4 Production
- 4.1 Laboratory
- 4.2 Industrial
- 4.3 Thermochemical
- 4.4 Anaerobic corrosion
- 4.5 Geological occurrence: the serpentinization reaction
- 4.6 Formation in transformers
5 Applications
- 5.1 Consumption in processes
- 5.2 Coolant
- 5.3 Energy carrier
- 5.4 Semiconductor industry
6 Biological reactions
7 Safety and precautions
8 See also
9 Notes
10 References
11 Further reading
12 External links

Properties

Combustion

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.

Hydrogen gas (dihydrogen or molecular hydrogen)^[12] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.^[13] The enthalpy of combustion for hydrogen is −286 kJ/mol:^[14]

2 H₂(g) + O₂(g) → 2 H₂O(l) + 572 kJ (286 kJ/mol)^{[note 2]}

Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures may be ignited by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).^[15] Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames.^[16] The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible orange flames were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.

H₂ reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids.^[17]

Electron energy levels

Main article: Hydrogen atom

Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale).

The ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm wavelength.^[18]

The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravity. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.^[19]

A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.^[20] The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all— an illustration of how different the "planetary orbit" conception of electron motion differs from reality.

Elemental molecular forms

Phases

Compressed hydrogen
Liquid hydrogen
Slush hydrogen
Solid hydrogen
Metallic hydrogen

Compounds

Further information: Hydrogen compounds

Covalent and organic compounds

While H₂ is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more electronegative, such as halogens (e.g., F, Cl, Br, I), or oxygen; in these compounds hydrogen takes on a partial positive charge.^[28] When bonded to fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding called hydrogen bonding, which is critical to the stability of many biological molecules.^[29]^[30] Hydrogen also forms compounds with less electronegative elements, such as the metals and metalloids, in which it takes on a partial negative charge. These compounds are often known as hydrides.^[31]

Hydrogen forms a vast array of compounds with carbon called the hydrocarbons, and an even vaster array with heteroatoms that, because of their general association with living things, are called organic compounds.^[32] The study of their properties is known as organic chemistry^[33] and their study in the context of living organisms is known as biochemistry.^[34] By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.^[32] Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways, which seldom involve elementary hydrogen.

Hydrides

Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H⁻, and is used when hydrogen forms a compound with a more electropositive element. The existence of the hydride anion, suggested by Gilbert N. Lewis in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 by the electrolysis of molten lithium hydride (LiH), producing a stoichiometry quantity of hydrogen at the anode.^[35] For hydrides other than group I and II metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is BeH
2, which is polymeric. In lithium aluminium hydride, the AlH−
4 anion carries hydridic centers firmly attached to the Al(III).

Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminium hydride.^[36] Binary indium hydride has not yet been identified, although larger complexes exist.^[37]

In inorganic chemistry, hydrides can also serve as bridging ligands that link two metal centers in a coordination complex. This function is particularly common in group 13 elements, especially in boranes (boron hydrides) and aluminium complexes, as well as in clustered carboranes.^[38]

Protons and acids

Further information: Acid–base reaction

Oxidation of hydrogen removes its electron and gives H⁺, which contains no electrons and a nucleus which is usually composed of one proton. That is why H+
is often called a proton. This species is central to discussion of acids. Under the Bronsted-Lowry theory, acids are proton donors, while bases are proton acceptors.

A bare proton, H+
, cannot exist in solution or in ionic crystals, because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the electron clouds of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or cationic hydrogen attached to other species in this fashion, and as such is denoted "H+
" without any implication that any single protons exist freely as a species.

To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "hydronium ion" (H
3O+
). However, even in this case, such solvated hydrogen cations are thought more realistically physically to be organized into clusters that form species closer to H
9O+
4.^[39] Other oxonium ions are found when water is in solution with other solvents.^[40]

Although exotic on Earth, one of the most common ions in the universe is the H+
3 ion, known as protonated molecular hydrogen or the trihydrogen cation.^[41]

Isotopes

Main article: Isotopes of hydrogen

Hydrogen discharge (spectrum) tube

Deuterium discharge (spectrum) tube

Protium, the most common isotope of hydrogen, has one proton and one electron. Unique among all stable isotopes, it has no neutrons (see diproton for a discussion of why others do not exist).

Hydrogen has three naturally occurring isotopes, denoted 1
H, 2
H and 3
H. Other, highly unstable nuclei (4
H to 7
H) have been synthesized in the laboratory but not observed in nature.^[42]^[43]

1
H is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.^[44]
2
H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus. Essentially all deuterium in the universe is thought to have been produced at the time of the Big Bang, and has endured since that time. Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1
H-NMR spectroscopy.^[45] Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.^[46]
3
H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32 years.^[38] It is so radioactive that it can be used in luminous paint, making it useful in such things as watches. The glass prevents the small amount of radiation from getting out.^[47] Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases; tritium has also been released during nuclear weapons tests.^[48] It is used in nuclear fusion reactions,^[49] as a tracer in isotope geochemistry,^[50] and specialized in self-powered lighting devices.^[51] Tritium has also been used in chemical and biological labeling experiments as a radiolabel.^[52]

Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritium. The symbols D and T (instead of 2
H and 3
H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for phosphorus and thus is not available for protium.^[53] In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry allows any of D, T, 2
H, and 3
H to be used, although 2
H and 3
H are preferred.^[54]

History

Discovery and use

Main article: Timeline of hydrogen technologies

In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.^[55]^[56] In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "flammable air". He speculated that "flammable air" was in fact identical to the hypothetical substance called "phlogiston"^[57]^[58] and further finding in 1781 that the gas produces water when burned. He is usually given credit for its discovery as an element.^[2]^[3] In 1783, Antoine Lavoisier gave the element the name hydrogen (from the Greek ὕδρω hydro meaning water and γενῆς genes meaning creator)^[4] when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.^[3]

Antoine-Laurent de Lavoisier

Lavoisier produced hydrogen for his famous experiments on mass conservation by reacting a flux of steam with metallic iron through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:

Fe + H₂O → FeO + H₂

2 Fe + 3 H₂O → Fe₂O₃ + 3 H₂

3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂

Many metals such as zirconium undergo a similar reaction with water leading to the production of hydrogen.

Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.^[3] He produced solid hydrogen the next year.^[3] Deuterium was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.^[2] Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.^[3] François Isaac de Rivaz built the first internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819. The Döbereiner's lamp and limelight were invented in 1823.^[3]

The first hydrogen-filled balloon was invented by Jacques Charles in 1783.^[3] Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard.^[3] German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called Zeppelins; the first of which had its maiden flight in 1900.^[3] Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war.

The first non-stop transatlantic crossing was made by the British airship R34 in 1919. Regular passenger service resumed in the 1920s and the discovery of helium reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, H₂ was used in the Hindenburg airship, which was destroyed in a midair fire over New Jersey on May 6, 1937.^[3] The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the aluminized fabric coating by static electricity. But the damage to hydrogen's reputation as a lifting gas was already done.

In the same year the first hydrogen-cooled turbogenerator went into service with gaseous hydrogen as a coolant in the rotor and the stator in 1937 at Dayton, Ohio, by the Dayton Power & Light Co,^[59] because of the thermal conductivity of hydrogen gas this is the most common type in its field today.

The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).^[60] For example, the ISS,^[61] Mars Odyssey^[62] and the Mars Global Surveyor^[63] are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009, more than 19 years after launch, and 13 years over their design life.^{[citation needed]}

Role in quantum theory

Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series

Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure.^[64] Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H+
2 allowed fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.

One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogen, half a century before full quantum mechanical theory arrived. Maxwell observed that the specific heat capacity of H₂ unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H₂ because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.^[65]

Natural occurrence

NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy

Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms (most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as dark matter and dark energy).^[66] This element is found in great abundance in stars and gas giant planets. Molecular clouds of H₂ are associated with star formation. Hydrogen plays a vital role in powering stars through proton-proton reaction and CNO cycle nuclear fusion.^[67]

Throughout the universe, hydrogen is mostly found in the atomic and plasma states whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. Hydrogen is found in the neutral atomic state in the interstellar medium. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the Universe up to redshift z=4.^[68]

Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H₂ (for data see table^[ambiguous]). However, hydrogen gas is very rare in the Earth's atmosphere (1 ppm by volume) because of its light weight, which enables it to escape from Earth's gravity more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface,^[69] mostly in the form of chemical compounds such as hydrocarbons and water.^[38] Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, as is methane, itself a hydrogen source of increasing importance.^[70]

A molecular form called protonated molecular hydrogen (H+
3) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from cosmic rays. This charged ion has also been observed in the upper atmosphere of the planet Jupiter. The ion is relatively stable in the environment of outer space due to the low temperature and density. H+
3 is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium.^[71] Neutral triatomic hydrogen H₃ can only exist in an excited form and is unstable.^[72] By contrast, the positive hydrogen molecular ion (H+
2) is a rare molecule in the universe.

Production

For more details on this topic, see Hydrogen production.

H₂ is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the hydrogenation of unsaturated substrates; and in nature as a means of expelling reducing equivalents in biochemical reactions.

Laboratory

In the laboratory, H₂ is usually prepared by the reaction of dilute non-oxidizing acids on some reactive metals such as zinc with Kipp's apparatus.

Zn + 2 H+
→ Zn2+
+ H
2

Aluminium can also produce H
2 upon treatment with bases:

2 Al + 6 H
2O + 2 OH−
→ 2 Al(OH)−
4 + 3 H
2

The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is in the range 80–94%.^[73]

2 H
2O(l) → 2 H
2(g) + O
2(g)

In 2007, it was discovered that an alloy of aluminium and gallium in pellet form added to water could be used to generate hydrogen. The process also creates alumina, but the expensive gallium, which prevents the formation of an oxide skin on the pellets, can be re-used. This has important potential implications for a hydrogen economy, as hydrogen can be produced on-site and does not need to be transported.^[74]

Industrial

Main article: Hydrogen production

Hydrogen can be prepared in several different ways, but economically the most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas.^[75] At high temperatures (1000–1400 K, 700–1100 °C or 1300–2000 °F), steam (water vapor) reacts with methane to yield carbon monoxide and H
2.

CH
4 + H
2O → CO + 3 H
2

This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 MPa, 20 atm or 600 inHg). This is because high-pressure H
2 is the most marketable product and Pressure Swing Adsorption (PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and related compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:

CH
4 → C + 2 H₂

Consequently, steam reforming typically employs an excess of H
2O. Additional hydrogen can be recovered from the steam by use of carbon monoxide through the water gas shift reaction, especially with an iron oxide catalyst. This reaction is also a common industrial source of carbon dioxide:^[75]

CO + H
2O → CO
2 + H
2

Other important methods for H
2 production include partial oxidation of hydrocarbons:^[76]

2 CH
4 + O
2 → 2 CO + 4 H
2

and the coal reaction, which can serve as a prelude to the shift reaction above:^[75]

C + H
2O → CO + H
2

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia, hydrogen is generated from natural gas.^[77] Electrolysis of brine to yield chlorine also produces hydrogen as a co-product.^[78]

Thermochemical

There are more than 200 thermochemical cycles which can be used for water splitting, around a dozen of these cycles such as the iron oxide cycle, cerium(IV) oxide-cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricity.^[79] A number of laboratories (including in France, Germany, Greece, Japan, and the USA) are developing thermochemical methods to produce hydrogen from solar energy and water.^[80]

Anaerobic corrosion

Under anaerobic conditions, iron and steel alloys are slowly oxidized by the protons of water concomitantly reduced in molecular hydrogen (H₂). The anaerobic corrosion of iron leads first to the formation of ferrous hydroxide (green rust) and can be described by the following reaction:

Fe + 2 H₂O → Fe(OH)₂ + H₂

In its turn, under anaerobic conditions, the ferrous hydroxide (Fe(OH)₂ ) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:

3 Fe(OH)₂ → Fe₃O₄ + 2 H₂O + H₂

ferrous hydroxide → magnetite + water + hydrogen

The well crystallized magnetite (Fe₃O₄) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)₂ ).

This process occurs during the anaerobic corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.

Geological occurrence: the serpentinization reaction

In the absence of atmospheric oxygen (O₂), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H₂) is produced during the process of serpentinization by the anaerobic oxidation by the water protons (H⁺) of the ferrous (Fe²⁺) silicate present in the crystal lattice of the fayalite (Fe₂SiO₄, the olivine iron-endmember). The corresponding reaction leading to the formation of magnetite (Fe₃O₄), quartz (SiO₂) and hydrogen (H₂) is the following:

3 Fe₂SiO₄ + 2 H₂O → 2 Fe₃O₄ + 3 SiO₂ + 3 H₂

fayalite + water → magnetite + quartz + hydrogen

This reaction closely resembles the Schikorr reaction observed in the anaerobic oxidation of the ferrous hydroxide in contact with water.

Formation in transformers

From all the fault gases formed in power transformers, hydrogen is the most common and is generated under most fault conditions; thus, formation of hydrogen is an early indication of serious problems in the transformer's life cycle.^[81]

Applications

Consumption in processes

Large quantities of H
2 are needed in the petroleum and chemical industries. The largest application of H
2 is for the processing ("upgrading") of fossil fuels, and in the production of ammonia. The key consumers of H
2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocracking. H
2 has several other important uses. H
2 is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and oils (found in items such as margarine), and in the production of methanol. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H
2 is also used as a reducing agent of metallic ores.^[82]

Hydrogen is highly soluble in many rare earth and transition metals^[83] and is soluble in both nanocrystalline and amorphous metals.^[84] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.^[85] These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is a metallurgical problem, contributing to the embrittlement of many metals,^[10] complicating the design of pipelines and storage tanks.^[11]

Apart from its use as a reactant, H
2 has wide applications in physics and engineering. It is used as a shielding gas in welding methods such as atomic hydrogen welding.^[86]^[87] H₂ is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas. Liquid H₂ is used in cryogenic research, including superconductivity studies.^[88] Because H
2 is lighter than air, having a little more than ¹⁄₁₄ of the density of air, it was once widely used as a lifting gas in balloons and airships.^[89]

In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called forming gas) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries.^[90] Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.^[91]

Hydrogen's rarer isotopes also each have specific applications. Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions.^[3] Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.^[92] Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs,^[93] as an isotopic label in the biosciences,^[52] and as a radiation source in luminous paints.^[94]

The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale at 13.8033 kelvins.^[95]

Coolant

Main article: Hydrogen-cooled turbo generator

Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low density, low viscosity, and the highest specific heat and thermal conductivity of all gases.

Energy carrier

See also: Hydrogen economy and Hydrogen infrastructure

Hydrogen is not an energy resource,^[96] except in the hypothetical context of commercial nuclear fusion power plants using deuterium or tritium, a technology presently far from development.^[97] The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth.^[98] Elemental hydrogen from solar, biological, or electrical sources require more energy to make it than is obtained by burning it, so in these cases hydrogen functions as an energy carrier, like a battery. Hydrogen may be obtained from fossil sources (such as methane), but these sources are unsustainable.^[96]

The energy density per unit volume of both liquid hydrogen and compressed hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher.^[96] Nevertheless, elemental hydrogen has been widely discussed in the context of energy, as a possible future carrier of energy on an economy-wide scale.^[99] For example, CO
2 sequestration followed by carbon capture and storage could be conducted at the point of H
2 production from fossil fuels.^[100] Hydrogen used in transportation would burn relatively cleanly, with some NO_x emissions,^[101] but without carbon emissions.^[100] However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.^[102]

Semiconductor industry

Hydrogen is employed to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon that helps stabilizing material properties.^[103] It is also a potential electron donor in various oxide materials, including ZnO,^[104]^[105] SnO₂, CdO, MgO,^[106] ZrO₂, HfO₂, La₂O₃, Y₂O₃, TiO₂, SrTiO₃, LaAlO₃, SiO₂, Al₂O₃, ZrSiO₄, HfSiO₄, and SrZrO₃.^[107]

Biological reactions

Further information: Biohydrogen and Biological hydrogen production

H₂ is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H₂ and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water.^[108]

Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H₂ gas by specialized hydrogenases in the chloroplast.^[109] Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H₂ gas even in the presence of oxygen.^[110] Efforts have also been undertaken with genetically modified alga in a bioreactor.^[111]

Safety and precautions

Main article: Hydrogen safety

Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.^[112] In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids.^[113] Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement,^[114] leading to cracks and explosions.^[115] Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.^[116]

Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the parahydrogen/orthohydrogen ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given). Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.^[112]

Notes

^ However, most of the universe's mass is not in the form of baryons or chemical elements. See dark matter and dark energy.
^ 286 kJ/mol: energy per mole of the combustible material (hydrogen)

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★リンクテーブル★

リンク元	「過酸化水素」
拡張検索	「hydrogensulfite reductase」「hydroxybutyrate dehydrogenase」「formate dehydrogenase」

「過酸化水素」

Library resources
About Hydrogen
	Online books; Resources in your library; Resources in other libraries;

Hydrogen
	1H
	- ← hydrogen → helium
	Hydrogen in the periodic table
Appearance
	colorless gas; ; Purple glow in its plasma state; ; Spectral lines of hydrogen
General properties
Name, symbol, number	hydrogen, H, 1
Pronunciation	/ˈhaɪdrədʒən/ HY-drə-jən
Element category	diatomic nonmetal
Group, period, block	1, 1, s
Standard atomic weight	1.008(1)
Electron configuration	1s1; 1
History
Discovery	Henry Cavendish (1766)
Named by	Antoine Lavoisier (1783)
Physical properties
Color	colorless
Phase	gas
Density	(0 °C, 101.325 kPa); 0.08988 g/L
Liquid density at m.p.	0.07 (0.0763 solid) g·cm−3
Liquid density at b.p.	0.07099 g·cm−3
Melting point	13.99 K, -259.16 °C, -434.49 °F
Boiling point	20.271 K, -252.879 °C, -423.182 °F
Triple point	13.8033 K, 7.041 kPa
Critical point	32.938 K, 1.2858 MPa
Heat of fusion	(H2) 0.117 kJ·mol−1
Heat of vaporization	(H2) 0.904 kJ·mol−1
Molar heat capacity	(H2) 28.836 J·mol−1·K−1
Vapor pressure
Atomic properties
Oxidation states	1, -1; (amphoteric oxide)
Electronegativity	2.20 (Pauling scale)
Ionization energies	1st: 1312.0 kJ·mol−1
Covalent radius	31±5 pm
Van der Waals radius	120 pm
Miscellanea
Crystal structure	hexagonal
Magnetic ordering	diamagnetic
Thermal conductivity	0.1805 W·m−1·K−1
Speed of sound	(gas, 27 °C) 1310 m·s−1
CAS registry number	1333-74-0
Most stable isotopes
	Main article: Isotopes of hydrogen
	v; t; e; · ref

外見
	無色の気体;
一般特性
名称, 記号, 番号	水素, H, 1
分類	非金属
族, 周期, ブロック	1, 1, s
原子量	1.00794(7) g·mol-1
電子配置	1s1
電子殻	1（画像）
物理特性
色	無色
相	気体
密度	(0 °C, 101.325 kPa); 0.08988 g/L
融点	14.01 K, −259.14 °C, −434.45 °F
沸点	20.28 K, −252.87 °C, −423.17 °F
三重点	13.8033 K (-259°C), 7.042 kPa
臨界点	32.97 K, 1.293 MPa
融解熱	(H2) 0.117 kJ·mol-1
蒸発熱	(H2) 0.904 kJ·mol-1
熱容量	(25 °C) (H2) 28.836 J·mol-1·K-1
蒸気圧
原子特性
酸化数	1, −1; (両性酸化物)
電気陰性度	2.20 (ポーリングの値)
イオン化エネルギー	1st: 1312.0 kJ·mol-1
共有結合半径	31±5 pm
ファンデルワールス半径	120 pm
その他
結晶構造	六方晶系
磁性	反磁性
熱伝導率	(300 K) 0.1805 W·m-1·K-1
音の伝わる速さ	(gas, 27 °C) 1310 m/s
CAS登録番号	1333-74-0
最安定同位体
	詳細は水素の同位体を参照
	表示

　　[★]

hydrogen peroxide
英: hydrogen peroxide
関

H₂O₂

白血球などでNADPH oxidaseによって産生される

「hydrogensulfite reductase」

　　[★] 亜硫酸水素還元酵素、亜硫酸水素レダクターゼ、重亜硫酸還元酵素、重亜硫酸レダクターゼ

「hydroxybutyrate dehydrogenase」

　　[★]

ヒドロキシ酪酸脱水素酵素、ヒドロキシ酪酸デヒドロゲナーゼ

関: 3-hydroxybutyrate dehydrogenase

「formate dehydrogenase」

　　[★]

葉酸脱水素酵素、葉酸デヒドロゲナーゼ

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[22] ハロゲンに近い性質を持つため、1周期系列と17族の位置に変更すべきというもの。

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[8] However, most of the universe's mass is not in the form of baryons or chemical elements. See dark matter and dark energy.

[16] 286 kJ/mol: energy per mole of the combustible material (hydrogen)

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